The present invention relates to a process of manufacturing a ceramic circuit board, wherein at least dielectric ceramics and a conductor paste of base metal are co-fired.
In the art of manufacture of ceramic circuit boards, especially multiple-layered ceramic circuit boards, a method has been known wherein conductor circuit patterns are screen-printed on a dielectric ceramic green body, with a conductor paste which includes, as a major component thereof, at least one noble metal such as gold, silver, platinum and their alloys that are stable in an oxidizing atmosphere. The ceramic green body and the printed conductor circuit patterns of noble metal paste are then co-fired in air, and thus a ceramic circuit board is obtained. An alternative method is known, wherein the conductor circuit patterns are formed of a conductor paste the major component of which is a base metal such as copper, nickel and tungsten, and wherein a non-oxidizing atmosphere is used for firing the ceramic green body and the printed conductor paste.
The former method of manufacturing a ceramic circuit board is advantageous in that the co-firing process may be achieved in air. However, this method can be practiced only at a high cost because of the use of gold, silver and other noble metals that are considerably expensive. Thus, the method suffers a serious problem in terms of manufacturing economy. On the other hand, the latter method which uses base metals as a conductor paste is advantageous in material cost and excellent in electrical properties of the obtained ceramic circuit board, but has some significant drawbacks in quality of the end products, i.e., ceramic circuit boards. Stated more specifically, since the firing operation in this latter method is effected in a non-oxidizing atmosphere, an organic binder which is a component of a dielectric ceramic composition forming an insulation layer of the ceramic circuit board, is not sufficiently decomposed in the non-oxidizing atmosphere, that is, the organic binder is left in the form of carbon, whereby the ceramic circuit board tends to have blackened surfaces, pin holes and blisters, and are liable to suffer insufficient sintering and deterioration of electrical properties.
As stated above, the known dielectric ceramic composition with base metal conductors has problems of low rate of burnout or removal of an organic binder, and low degree of sintering. In view of these problems, it has been considered difficult to industrially practice the method wherein a ceramic circuit board is fabricated by way of co-firing the ceramic dielectric composition including a glass component, together with base metal conductors in a non-oxidizing atmosphere.
Nevertheless, efforts have been made to overcome the above indicated problems associated with the technology of co-firing a dielectric ceramic composition with a conductor of base metal. For example, various ceramic compositions for dielectrics have been proposed, including: a ceramic composition containing crystallizable glass; a ceramic composition containing a relatively large amount of alumina, zirconia or other refractory filler which is mixed with a glass component; a ceramic composition containing an oxide of high valence metal such as Co.sub.3 O.sub.4 ; and a ceramic composition containing an organic binder which is thermally decomposable at a low temperature. However, these proposed compositions have not been found satisfactory to eliminate the drawbacks experienced in the prior art.